This invention relates to portable image scanners in general and more specifically to a portable scanner navigation system having a variable aperture.
Optical scanner devices are well-known in the art and produce machine-readable image data signals that are representative of a scanned object, such as a photograph or a page of printed text. In a typical scanner application, the image data signals produced by an optical scanner may be used by a personal computer to reproduce an image of the scanned object on a suitable display device, such as a CRT or a printer.
A hand-held or portable scanner is an optical scanner which is designed to be moved by hand across the object or document being scanned. The hand-held scanner may be connected directly to a separate computer by a data cable. If so, the data signals produced by the hand-held scanner may be transferred to the separate computer xe2x80x9con the fly,xe2x80x9d i.e., as the image data are collected. Alternatively, the portable hand-scanner may include an on-board data storage system for storing the image data. The image data may then be uploaded to a separate computer after the scanning operation is complete by any convenient means.
Hand-held or portable optical scanners are well-known in the art and various components thereof are disclosed in U.S. Pat. No. 5,552,597 of McConica for xe2x80x9cHand-Held Scanner having Adjustable Light Pathxe2x80x9d, U.S. Pat. No. 5,586,212 of McConica, et al., for xe2x80x9cOptical Wave Guide for Hand-Held Scanner,xe2x80x9d U.S. Pat. No. 5,381,020 of Kochis, et al., for xe2x80x9cHand-Held Optical Scanner with Onboard Battery Recharging Assembly,xe2x80x9d and U.S. Pat. No. 5,306,908 of McConica, et al., for xe2x80x9cManually Operated Hand-Held Optical Scanner with Tactile Speed Control Assembly,xe2x80x9d all of which are hereby incorporated by reference for all that they disclose.
A typical hand-held optical scanner may include illumination and optical systems to accomplish scanning of the object. The illumination system illuminates a portion of the object (commonly referred to as a xe2x80x9cscan regionxe2x80x9d), whereas the optical system collects light reflected by the illuminated scan region and focuses a small area of the illuminated scan region (commonly referred to as a xe2x80x9cscan linexe2x80x9d) onto the surface of a photosensitive detector positioned within the scanner. Image data representative of the entire object then may be obtained by sweeping the scan line across the entire object, usually by moving the hand-held scanner with respect to the object. By way of example, the illumination system may include a light source (e.g., a fluorescent or incandescent lamp or an array of light emitting diodes (LEDs)). The optical system may include a lens and/or mirror assembly to direct and focus the image of the illuminated scan line onto the surface of the detector. The optical system may also comprise a reduction optics system for reducing the reflected light onto a very small detector.
The photosensitive detector used to detect the image light focused thereon by the optical system may be a contact image sensor, an array of photodetectors which act together to capture the image of a scan line. A typical individual photodetector changes electrical capacitance as it is exposed to light. As the intensity of the light reflected from the image onto the photodetector changes, the electrical current passing through the photodetector varies. Thus a photodetector may be used to detect light and dark regions on an image.
The term xe2x80x9cimage lightxe2x80x9d as used herein refers to the light that is focused onto the surface of the detector array by the optical system. Depending on the type of scanner and the type of document, the image light may be reflected from the document or object being scanned or it may be transmitted through the object or document. The image light may be converted into digital signals, or image data, in essentially three steps. First, each photodetector converts the light it receives into an electric charge. Second, the charges from the photodetectors are converted into analog voltages by an analog amplifier. Finally, the analog voltages are digitized by an analog-to-digital (A/D) converter. The digital image data then may be processed and/or stored as desired.
A typical portable scanner may also include a position sensing system, or navigation system, to keep track of the scanner""s position with respect to the object. The navigation system allows a portable scanner to image an object or document which is larger and wider than the scanner by combining scanned strips or swaths into a single image. As the portable scanner is moved back and forth across the object to scan all areas of the object, the navigation system keeps track of the position of the scanner. The various scanned swaths may then be xe2x80x9cstitchedxe2x80x9d together using the position information collected by the position sensing system.
One type of navigation system employs optical detectors which generate image data representing a two-dimensional portion of the surface of the object. The optical detectors capture the image of several target locations on the object. An optical lens and a fixed aperture are positioned adjacent each optical detector to focus and direct light reflected from the object onto the optical detectors. A processor analyzes the image data to identify the positions of distinct features located on the surface of the object relative to the optical sensor. As the scanner is moved relative to the object, the positions of these distinct features relative to the optical sensor move accordingly. The processor measures these position changes to determine the displacement and direction of movement of the scanning device relative to the surface of the object. The processor may also integrate the displacement to determine the velocity of the scanner relative to the surface of the object. Examples of position sensors, or navigators, that image two-dimensional areas of an object are disclosed in U.S. Pat. No. 5,644,139 of Allen, et al., for xe2x80x9cNavigation Technique for Detecting Movement of Navigation Sensors Relative to an Object,xe2x80x9d and U.S. Pat. No. 5,578,813 of Allen, et al., for xe2x80x9cFreehand Image Scanning Device Which Compensates for Non-linear Movement,xe2x80x9d both of which are hereby incorporated by reference for all that they disclose.
Portable scanners of the type described above are not without their problems. For example, the efficacy of the navigation system varies greatly according to the content and material of the object. Factors affecting the navigation system include the color of the object, texture, coating, and content. White printer paper typically has a reflectance of about 80%, while black glossy magazine pages have a reflectance of less than about 3%. Glossy objects also have a large specular component to the reflectance, requiring a grazing illumination with a large angle of incidence on the object. Thus the amount of light reflected from the object to the navigator optical detector is much higher for a white object than for a black object.
The fixed size of the aperture in a navigator is typically determined by balancing the requirements for the various types of objects that will be scanned. A further consideration is the depth of field required by the navigator. The smaller the aperture, the greater the depth of field. Thus, a portable scanner with a smaller aperture is less susceptible to focus errors caused by tilting the scanner over the object. However, the fixed aperture does not provide an ideal amount of light to the navigator optical detector for all object types. Therefore, a portable scanner navigation system which performs well on white printer paper may be unable to adequately track scanner movement on black glossy magazine paper.
A need therefore exists for a portable scanner navigation system which is more effective when used with varied types of objects with varied content. A further need exists for a portable scanner navigation system which can optimize the depth of field for varied types of objects.
To assist in achieving the aforementioned need, the inventor has devised a portable scanner having a navigation system with a variable aperture. The aperture size is varied according to the type of object to be scanned, increasing the efficacy of the navigation system over a range of object types.
The invention may comprise a portable scanner for document imaging having at least one position detector. The at least one position detector comprises an optical detector and an aperture assembly adjacent the optical detector. The aperture assembly has a first aperture size and a second aperture size. The aperture assembly is adjustable between the first aperture size and the second aperture size.
The invention may also comprise a method for varying the intensity of light incident on a navigator photodetector in a portable scanner. The method comprises providing an aperture assembly adjacent the navigator photodetector in the portable scanner, wherein the aperture assembly has a variable aperture size which may be adjusted to vary the intensity of light incident on the navigator photodetector. The method also comprises determining an optimal aperture size which best enables the navigator photodetector to detect a position of the portable scanner. The method also comprises adjusting the aperture assembly so that the variable aperture size is set substantially at the optimal aperture size.
The invention may also comprise a portable scanner for document imaging, comprising at least one position detector, an optical detector located in the position detector, and means for varying a cross-sectional area of a light path to the optical detector.